[TYPO3-Solr] exception Apache_Solr_HttpTransportException with message 500

[Lars Peipmann]

Could solr install almost successful. Site titles are indexed, content 
elements unfortunately not.

Solr is available with http://localhost:8080/solr/ therefore i didn't 
changed the extension constants.

curl http://host:port/path-to-solr/update -H "Content-Type: text/xml" 
--data-binary '<commit />' prints this xml response:
<?xml version="1.0" encoding="UTF-8"?>
<response>
<lst name="responseHeader"><int name="status">0</int><int 
name="QTime">34</int></lst>
</response>

Developer Log shows this Exception:
exception 'Apache_Solr_HttpTransportException' with message ''500' 
Status: Internal Server Error' in 
/var/www/common/typo3/extensions/solr_trunk/lib/SolrPhpClient/Apache/Solr/Service.php:441
Stack trace:
#0 
/var/www/common/typo3/extensions/solr_trunk/lib/SolrPhpClient/Apache/Solr/Service.php(734): 
Apache_Solr_Service->_sendRawPost('http://localhos...', '<add 
allowDups=...')
#1 
/var/www/common/typo3/extensions/solr_trunk/lib/SolrPhpClient/Apache/Solr/Service.php(788): 
Apache_Solr_Service->add('<add allowDups=...')
#2 
/var/www/common/typo3/extensions/solr_trunk/classes/class.tx_solr_indexer.php(239): 
Apache_Solr_Service->addDocuments(Array)
#3 
/var/www/common/typo3/extensions/solr_trunk/classes/class.tx_solr_indexer.php(90): 
tx_solr_Indexer->indexPage(18)
#4 
/var/www/common/typo3/trunk/typo3/sysext/cms/tslib/class.tslib_fe.php(3175): 
tx_solr_Indexer->hook_indexContent(Object(tslib_fe))
#5 /var/www/common/typo3/trunk/typo3/sysext/cms/tslib/index_ts.php(452): 
tslib_fe->generatePage_postProcessing()
#6 /var/www/common/typo3/trunk/index.php(84): require('/var/www/common...')
#7 {main}

Any ideas where my mistake could be?